Copper transmission link technologies such as Digital Subscriber Line (xDSL) are providing, as of today, access broadband services to 286 million subscribers worldwide. Different generations of DSL technology, such as Asymmetric DSL (ADSL), ADSL2(+), Very high bitrate DSL (VDSL) and VDLS2 provide data rates in the range from several Mb/s up to 100 Mb/s over ranges from 1 km to 8 km. Recently, the need for Gigabit speeds on telephone-grade copper has arisen for broadband access, home networking, as well as 4G mobile network backhaul, such as e.g. LTE S1/X2 interface backhaul.
New generations of DSL-like systems can provide this capacity on very short lines/loops in the area of 50-200 meters. Such loops provide 100 to 200 MHz of bandwidth for data transmission, as compared to earlier maximum bandwidths of about 30 MHz for legacy systems. Unlike classical DSL systems transmitting uplink and downstream data in different bands of the copper in a frequency-division duplexing scheme (FDD), Gigabit DSL utilizes more hardware-friendly time-division-duplexing (TDD), where upstream and downstream data is utilizing the whole copper spectrum in a time-shared manner—i.e. the transceiver either transmits or receives at a given point in time.
Block transmission using the Fast Fourier Transform (FFT) and its inverse, IFFT, for modulation and demodulation, respectively, is the dominating modulation scheme in today's communication systems. This modulating scheme is often referred to as multicarrier modulation. One of the two most important variants of multicarrier modulation is passband transmission using complex-valued transmit/receive signals, which is referred to as orthogonal frequency division multiplexing (OFDM). OFDM is used, for example, in wireless communication systems, such as LTE. The second one is baseband transmission using real-valued transmit/receive signals, which is referred to as DMT. DMT is used, for example, in wireline communication systems, such as xDSL systems using e.g. copper cables.
Simultaneous transmission and reception of signals requires a scheme for separating the two signals. Separation in time, also referred to as TDD, is a suitable method for low-complexity, and thus low-cost, transceiver implementations. The cost can be kept low, e.g. since there is a reduced need for echo cancellation when using TDD, as compared to when using frequency division. Examples of TDD communication systems include e.g. transmission over any kind of copper transmission media, such as twisted pair, CAT5, etc. TDD systems may be used for various applications providing various services, such as e.g. Internet access and base-station backhaul. The communication may be, and is being, standardized in different variants, such as G.fast and G.hn, but may also be used in different non-standardized forms.
Time division duplexing (TDD) is a well-known transmission scheme in telecommunication systems such as ATM, GSM, LTE-TDD, and different flavors of Ethernet Passive Optical Network (EPON) and Gigabit-capable Passive Optical Networks (GPON). Thereby bi-directional transmission between a line terminal (LT) and a network terminal (NT) is accomplished by utilizing a common media for upstream and downstream transmission in non-overlapping instances of time, i.e. time-slots.
In previous generations of Digital Subscriber Line (DSL) technology, frequency division duplexing (FDD) was used, where non-overlapping frequency bands on the copper wire are used to transmit upstream and downstream simultaneously. Such a structure mitigates near-end crosstalk (NEXT) by design. In order to mitigate far-end crosstalk (FEXT) vectoring might be used to reach near-to channel capacity.
Recently, standardization of the new DSL standard in ITU-T, called G.fast, has started utilizing TDD instead of FDD in the physical layer to reduce system complexity and thereby cost. In contrary to FDD, TDD does have a NEXT problem if simultaneous transmission in opposite direction on wires of a common binder happens. It is the task of the framer to construct non-overlapping frames in time to prevent NEXT. FEXT may be cancelled by vectoring.
The FIG. 1 exemplifies NEXT collisions that can occur in TDD DSL systems. NEXT happens when, on local or remote side, a transmitter sends and a receiver receives at the same time. In frame N, transmission to NTA and NTB are equal in terms of downstream (DS) and upstream (US) partition, which may also be referred to as partitioning, of the whole frame. As no overlap occurs between US and DS communication, no NEXT is generated. In frame N+1, NTB transmits longer in downstream than NTA resulting in harmful near end cross-talk, NEXT.